XMLRPC several methods for check for capability before it exists

[markoheijnen]

When you check for the capability to edit a post it also applies for not existing posts.
So if a post ID doesn't exists it will also say you are not allowed to edit the post.

So I guess wp_get_single_post should be before current_user_can. So the right error message can be applied. Also the check on post_date for "no such post" is weird

[markoheijnen]

Added unit test cases for the method mw_getPost ​[UT599]

[markoheijnen]

wp_getPage does the same: ​[UT601]

[nacin]

20336.diff​ is the result of reviewing every meta cap in class-wp-xmlrpc-server.php. Could use a second look.

[nacin]

In [20637]:

Check for the existence of a post before calling current_user_can() with a meta cap on that object. fixes #20336.